Conventionally, drinking water treatment plants such as the one disclosed in Patent Document 1 are used in order to obtain purified water by filtering raw water from rivers, lakes, marshes, and ponds and the like. In the treatment plant described in Patent Document 1, a partitioning wall is provided in a treatment tank in which raw water is stored so as to divide this treatment tank into two compartments. One of the compartments is used as a microorganism treatment section that treats raw water by adding thereto activated carbon such as granulated activated carbon to whose surface, in particular, aerobic microorganisms are adhered. The other compartment is used as a membrane filtration section that filters raw water that is fed from the one compartment by means of a filtration membrane unit that is placed inside this compartment.
In the drinking water treatment plant disclosed in Patent Document 1, raw water from which coarse matter and the like has been removed is supplied to the microorganism treatment section via a separation tank that separates coarse matter and easily precipitated matter by means of precipitation, and a circulatory flow is formed in the raw water while the raw water is aerated by means of an aeration apparatus which is provided in a bottom portion of this microorganism treatment section. By then stirring the activated carbon into the raw water by means of this circulatory flow, organic matter and the like can be absorbed and removed from the raw water. Furthermore, the raw water which has been treated in this way in the microorganism treatment section is supplied to the membrane filtration section by overflowing the top of the partitioning wall, and undergoes filtration treatment by being filtered by means of a filtration membrane unit that is formed by a plurality of hollow flat membrane modules. The filtered water which is obtained in this manner is then supplied as pure water to a storage tank which is provided outside the treatment tank.
An activated carbon outflow prevention baffle plate (referred to below simply as a ‘baffle plate’) is provided on a top end portion of a surface of the partitioning wall that faces towards the microorganism treatment section side. This baffle plate protrudes towards the inner side of the microorganism treatment section, and is provided so as to slope diagonally downwards towards a distal end thereof. As a result, the activated carbon which is being stirred by means of the circulatory flow in the microorganism treatment section collides with the baffle plate and is restricted or prevented from rising any further. Moreover, any activated carbon which moves above the baffle plate falls onto the top surface of the baffle plate due to its specific gravity being greater than that of the raw water, and drops into the interior of the microorganism treatment section because of the slope of the baffle plate and is recovered. Accordingly, in this drinking water treatment plant, although a structure is employed in which raw war is supplied to the membrane filtration section from the microorganism treatment section by overflowing the top of the partitioning wall, activated carbon from the microorganism treatment section generally does not flow into the filtration membrane section together with the overflowing raw water.    Patent Document 1: Japanese Patent No. 3491122